Changing filename extension

This is a discussion on Changing filename extension within the C Programming forums, part of the General Programming Boards category; Hi, My program takes two (filename) arguments, and I want the second argument to change to filename2C.dat [slippy@slipnslidn ~]% ./program ...

  1. #1
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    Changing filename extension

    Hi,

    My program takes two (filename) arguments, and I want the second argument to change to filename2C.dat

    [slippy@slipnslidn ~]% ./program filename1.dat filename2.dat

    Code:
     
    FILE* fn, fm, fo;
    
    int 
    main(int* argc, char** argv){
    
    fn = fopen(argv[1], "r");
    fm = fopen(argv[2], "r");
    
    /* This doesn't work... */
    
    fo = fopen(strcat(strstr(argv[2], "."), "C.dat", "w"));
    
    /* Do something */
    
    /* Shut things down */
    
    close(fn);
    close(fm);
    close(fo);
    }
    Thanks for your help.

  2. #2
    Registered User hk_mp5kpdw's Avatar
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    You should copy the command-line argument to a temp buffer and work on that, not on the argument directly. Aside from that, the strcat should probably be a strcpy instead and one of the right parenthesis is in the wrong place... it should be after the "C.Dat" string and not after the "w".
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  3. #3
    and the hat of wrongness Salem's Avatar
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    > strcat(strstr(argv[2], "."), "C.dat", "w")
    Split this out into it's component steps.
    For starters, strstr() can return NULL, which leaves you up the creek without a paddle.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  4. #4
    Captain Crash brewbuck's Avatar
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    That won't work because it uses memory improperly. You need to allocate enough space for the string and built it yourself:

    Code:
    #include <string.h>
    #include <stdlib.h>
    #include <stdio.h>
    
    char *dot, *newname;
    int len;
    
    /* Get position of the last dot in the name */
    dot = strrchr(name, '.');
    /* If a dot was found, calculate the length to this point */
    if(dot) len = dot - name;
    else len = strlen(name);
    /* Allocate a buffer big enough to hold the result.
     * 6 extra characters: 5 for "C.dat", 1 for null byte */
    newname = malloc(len + 6);
    /* Build the new name */
    strncpy(newname, name, len);
    strcpy(newname + len, "C.dat");

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